Mass spectrometry has been used for many decades in the characterization of small organic molecules. The technique typically involves the ionization of molecules in the sample to form molecular ions by subjecting the sample to an electron beam at a very low pressure. The molecular ions are then focused and accelerated by an electric field into a magnetic field or quadrupole. The ions are separated in the magnetic field or quadrupole according to the ratio of the mass of the ion m to the charge on the ion z (m/z). After passing through the field, the ions impinge upon a detector which determines the intensity of the ion beam and the m/z ratio, and these data are used to create the mass spectrum of the sample.
With the increasing interest in larger molecules, especially biomolecules such as nucleic acids and proteins, new techniques in the field of mass spectrometry are continually being developed to characterize these molecules.
In recent years the performance of commercially available mass spectrometers has seen significant improvement due, in part, to the availability of improved core components including more stable power supplies, faster digitizers, and more sophisticated fabrication methods for ion optic elements. Particularly noteworthy are the newest generation ESI-TOF mass spectrometers which, from several vendors in a variety of configurations, are now routinely yielding the types of mass measurement accuracy and mass resolution previously attainable only on high end sector or Fourier transform ion cyclotron resonance (FTICR)-based platforms. As such, the use of such bench top instruments by the bioanalytical community continues to expand as these instruments are increasingly being made available to scientists and technicians with a broad range of analytical needs. Accordingly, a number of increasingly sophisticated automation schemes are emerging, many incorporating some form of liquid chromatography (LC) as an on-line sample purification step to support high throughput QC or drug screening activities. While there are a number of applications in which some form of LC is a requisite step that facilitates the analysis of very complex mixtures, it is also used frequently as a generic desalting/purification protocol to prepare relatively pure analyte fractions for MS analysis.
Low molecular weight chemical noise is often the limiting factor in overall MS performance as the presence of high levels of low molecular weight components, such as polymers and buffer constituents, can drastically limit the spectral dynamic range and adversely affect mass accuracy. While LC is often used to reduce the adverse affects of such backgrounds, constraints on sample throughput and issues associated with solvent usage/disposal must be considered as part of the laboratory work flow. Additionally, LC is often used as a purification step (as opposed to a separation step) to render analytes amenable to MS analysis. Consequently, there is an increasing need for simple methods to reduce the chemical noise floor and render less than “pristine” samples amenable to mass spectrometric analysis.
The present invention satisfies this need, as well as others, by providing systems and methods for digital filtration of mass spectral signals arising from singly-charged low molecular weight components such as solution additives and matrix modifiers without significantly altering the mass spectral signals of larger analytes such as biomolecules.